THCOBRA: Ion back flow reduction in patterned THGEM cascades

A new concept for avalanche ion back-flow (IBF) reduction in cascaded gaseous detectors is presented. The Thick-COBRA is a Thick-GEM (THGEM) with a patterned electrode, aiming at reducing secondary effects by trapping avalanche ions. The patterned electrode faces the electron drift region, trapping ions flowing back from the cascaded multiplier and those produced within the Thick-COBRA. Total IBF values of about 5% were obtained, about six times below that reached with a THGEM in a triple-THGEM configuration, keeping full single-electron detection efficiency. Experimental and simulation studies of IBF and electron collection/transmission efficiencies are presented; the suitability for application of Thick-COBRA in photon detectors for RICH is discussed.